Dye diffusion thermal transfer printing (D2T2, also known as dye-sublimation or dye-sub), is a common digital printing technique in the ID card and security industry. To print a digital image using D2T2, a dye-sheet containing the dyes and a substrate known as a receiver is required. All receiver substrates must be able to accept the dyes during printing and to separate from the dyesheet without sticking. This can be a simple polymer that is able to accept dyes or a specially designed receiver coating that has been designed to function in a dye diffusion printer, as is the case in digital photography.
PVC is a widely used substrate in the card D2T2 printing industry due to its combination of properties: low cost; good dye receptivity; and it does not stick during printing. However, it has a relatively poor durability and is not the preferred substrate for other security printing techniques such as laser engraving. There are a number of other substrates of interest within the printing industry that offer certain advantages over PVC but there are various limitations to these alternative substrates.
In particular, polycarbonate (PC) is becoming a substrate of interest for card and ID applications, such as driver's licenses, student IDs or other ID cards, security badges, credit and debit cards and the like, due to its ability to be laser engraved and its general durability. However, PC cannot be directly printed by the common printing technique of dye diffusion thermal transfer (D2T2) printing. This is because the dye-sheet sticks to the PC surface during the high temperature, intimate contact of the D2T2 printing process, and the material also has poor dye receptivity for the dyes commonly used in D2T2 printing. The ability to diffuse small molecules from a polymer matrix is well known in the art as it forms the basic concept of D2T2 printing (see, e.g., European patent number EP0399690, the entire disclosure of which is herein incorporated by reference).
Accordingly, in D2T2 printing, a dye-sheet or ribbon is positioned in intimate contact with a substrate, on which it is desired to print a colored image. The dye-sheet or ribbon comprises a polyester (e.g. PET) substrate having a back coat, and a plurality of panels affixed in a suitable manner known in the art to the PET substrate on the side opposite the back coat. To aid adhesion of dye panels, an adhesive layer can be used on the PET base. This adhesive layer can be applied during base manufacture or coated onto PET prior to coating dye formulations.
Any mass transfer panel contained within a dye-sheet can be formulated such that it releases directly from PET base, or they can be applied on top of a releasing sub-coat. In the case of a base having a continuous adhesive applied prior to coating the panels, the releasing sub-coat may still be used.
A standard commercially available YMCKO dye-sheet or ribbon is illustrated in FIG. 1. As can be seen in FIG. 1, the five panels are affixed to the PET substrate, the last two removably affixed via a sub coat. These five panels in order are a yellow dye panel (Y), a magenta dye panel (M), a cyan dye panel (C), a black mass-transfer panel (K) and a mass transfer panel which can be used as a protective overlay (O), hence the acronym YMCKO. Ribbons may be of any format and panel length and other options are available, these include but are not limited to single color ribbons, YMC, YMCK, and YMCKOK. Other optional panels include but are not limited to diffusible security features, e.g., diffusive UV dyes, or other mass transfer features, such as UV pigments, metallic pigments, Optically Variable Pigments, taggants, etc. During D2T2 printing, the dye-sheet or ribbon is indexed over the substrate such that each of the panels is positioned over the substrate in succession. As is known in the art, a computer controlled thermal print head selectively heats each of the panels in the desired locations determined by a computer program to produce a colored image on the substrate.
The back coat on the PET substrate of the dye-sheet or ribbon aids in the transport across a thermal print head, and transfers heat from the thermal print head through to the dye-coats. When the yellow, magenta and cyan panels are heated, in turn, the respective colored dye is diffused from the panels at the locations where the heat is applied to produce the respective color on the substrate to form the image according to the computer image program. The amount of dye transferred is dependent on the temperature of the pixel of the print head. This is different from a mass transfer printing process where the transfer is either “on” or “off,” and an image is built up with a dithered pattern of colored dots. In the D2T2 process, 256 shades of each component color can be achieved and each component color can be blended with the other two component colors, giving a huge color gamut (16.7 million colors), and hence, continuous tone images are possible. The panels comprise a polymer and the dye, which when heated causes the dye to diffuse onto the substrate, while the polymer remains attached to the PET substrate of the dye-sheet or ribbon.
The black panel is attached to the PET substrate, for example via a sub-coat, which allows the black panel, comprising a polymer and a black pigment, to be transferred to the card substrate at the locations where heat is applied. Thus, unlike the yellow, magenta and cyan panels, both the polymer and the black pigment of the black panel are transferred to the substrate where heated. Similarly, the mass transferable protective overlay panel (O) is completely transferred from the dye-sheet or ribbon and onto the substrate where heated, typically over the entire substrate. Other panels are possible and may be included, which can function as either diffusion or mass transfer depending on the materials used. These panels can function as security features when using materials that are fluorescent under UV light, optically variable pigments, taggents, etc.
As is known, a PC substrate is a non-D2T2 printable substrate. When conventional D2T2 printing is tried on PC, only extremely low density images can be achieved, which have unacceptable low density. More commonly, the dye-sheet will stick during the printing operation, However, there are a number of alternative ways in which color images can be added to PC. For example, a color image could be added to PC by using a mass transfer printing technique, wherein a thermal transfer ribbon (TTR) is used to transfer the polymer and pigment onto the card (as opposed to D2T2 or dye-sublimation). Such a method has the limitation that the image will not be of the same quality as a D2T2 image due to the necessary dithering of the image, resulting in lower quality printing.
A color image could also be added to PC by pre-coating the PC with an image receiving layer such as a copolymer of polyvinyl chloride/vinyl acetate. A product of this type is commercially available from Bayer Materials known as Makrofol ID. This option has the limitation that a specific pre-coated card stock is required, which may be more expensive, and the entire upper surface of the card is no longer PC.
Another method of adding a color image to PC is by thermally transferring a receiver layer as a mass transfer patch, of any shape or size, from a ribbon onto a particular area of the substrate. FIG. 2 illustrates a D2T2 dye-sheet or ribbon used in this method. The general concept of a thermally transferable receiver layer is disclosed by a number of patents including U.S. Pat. No. 5,006,502 and EP0333873, JP62297184, and JP61084281. The disclosures of each of these patents are herein incorporated by reference. As shown in FIG. 2, a thermally transferable dye receptive layer or patch is removably attached to a D2T2 dye-sheet or ribbon, which can be via a sub-coat, in front of the yellow dye panel. As such, the first layer or patch, when heated, is transferred to the PC card substrate prior to D2T2 printing.
Subsequently, a colored image can be D2T2 printed onto this thermally transferable dye receptive layer or patch. This option leaves most of the card surface as PC, which has an advantage over the option of pre-coating the PC with an image receiving layer. However, a layer or patch of an alternative polymer is transferred to the PC card surface so there will still be a section of the PC substrate surface that is no longer 100% PC.
A fourth method of adding a color image to PC is by pre-treating PC sheets to enable them to be D2T2 printed. The concept of pre-treating the PC sheet surface is disclosed in U.S. Pat. No. 6,867,167 issued to Trueb. However, this is a treatment of the whole card surface. This option could allow a 100% PC substrate to be D2T2 printed, but the treatment step is unrelated to the personalization step and the image. As such, this option offers no added security, and the non-imaged areas of the card could be imaged at a later stage. This would potentially allow tampering of an ID document.
Therefore, a need exists for an improved method of adding a color image to a PC substrate via direct D2T2 printing, which is directed toward overcoming these and other disadvantages of prior art methods. Accordingly, to address the above stated issues, an improved method to render a PC substrate surface D2T2 printable at the point of personalization is needed. The exemplary teachings herein fulfill such a need. It is desired that the methods and techniques for providing the above benefits be applicable to any instances or applications wherein images are to be printed on a non-D2T2 printable surface.